A. Field of the Invention
The Invention is a compound aircraft that utilizes autorotation of the rotor to allow reduction in the rotational speed of the rotor during flight while avoiding instability of the rotor and excess deformation of the rotor blades due to aerodynamic forces resulting from the forward motion of the aircraft. The Invention is also a method of flight utilizing the compound aircraft of the invention.
B. Description of the Related Art
1. Compound Aircraft
A ‘compound’ aircraft is an aircraft that includes features of both fixed wing aircraft and rotary wing aircraft. The compound aircraft includes the elements of a helicopter, including at least one powered main rotor and a mechanism to overcome the torque reaction of the rotating main rotor. The compound aircraft also includes elements of a fixed-wing aircraft, such as a wing. The wing may be equipped with ailerons, flaps or a combination of flaps and ailerons known as ‘flaperons.’ The compound aircraft may be equipped with a separate thrust mechanism to drive the aircraft forward, such as a propeller in a ducted fan. Through the use of appropriate vanes or sectors that change the configuration of the duct, the ducted fan may serve as the mechanism to overcome the torque reaction of the rotating rotor blades and to provide yaw control.
The rotor blades of both a conventional helicopter and a compound aircraft are long and flexible compared to a fixed wing. A conventional helicopter and a compound aircraft both have operating limitations relating to the interaction of the forward speed of the aircraft and the rotation of the long, flexible blades. When a helicopter or compound aircraft is moving forward, a rotating rotor blade moving toward the front of the aircraft is ‘advancing.’ When the rotor blade is moving toward the rear of the aircraft, the blade is ‘retreating.’ As the forward speed of a helicopter or compound aircraft increases, the airspeed of an advancing rotor blade increases and the airspeed of a retreating blade decreases. As the local airspeed of the advancing blade approaches the speed of sound, shock waves occur that change the lift along the blade, apply torsion to the blade, increase drag and power requirements, and increase noise. These results of local airspeed on a helicopter or compound aircraft rotor blade are known collectively as ‘compressibility effects.’
As the speed of the aircraft in the forward direction increases, the local airspeed of the retreating rotor blade will decrease. As the forward speed of the aircraft continues to increase, the retreating blade progressively will lose lift and may eventually stall. To maintain lift generated by the retreating rotor and to prevent stalling, the rotational speed of the rotor must increase. Increasing the rotor speed to avoid retreating blade stall and loss of lift exacerbates the compressibility effects on the advancing blade.
These competing phenomena of ‘compressibility effects’ and ‘retreating blade stall’ limit the forward speed of helicopters and compound aircraft.
2. Autogyro
An autogyro is an aircraft that features a rotor and a propeller or other thruster that impels the aircraft in a forward direction. The autogyro differs from helicopters and compound aircraft in that the rotor is not powered. Air moving through the unpowered rotor disc from the underside of the rotor disc to the top of the rotor disc causes the rotor to rotate. The passively turning rotor provides lift to the aircraft. The autogyro rotor may be powered prior to takeoff to start the rotor turning. Any autogyro must move forward through the air to cause the rotor to turn and to allow the aircraft to maintain flight. An autogyro is not capable of a sustained hover in which the airspeed of the autogyro is zero.
The rotor of an autogyro passively spins in a stable manner due to a self-correcting balance of forces acting on the rotor blades due to the forward motion of the aircraft. Air passing through the rotor disc from below generates lift and drag forces that depend on the local angle of attack of the rotor blade, the rotational speed of the rotor and the forward speed of the aircraft. If the resultant force acting on a particular location of the rotor blade is ahead of the axis of rotation of the rotor, that portion of the rotor tends to speed up. If the local resultant force is behind the axis of rotation, that portion of the rotor tends to slow down. For a blade of constant pitch, the resultant forces along the inner portions of the blade closer to the hub will be ahead of those toward the outer portion of the blade closer to the tip. The forces acting on the inner portion of the blade therefore tend to accelerate the rotation of the blade, while the forces action on the outer portion of the blade tend to slow rotation of the blade.
The forces acting on the advancing and retreating blades also are different. A location on the retreating blade experiences a greater local angle of attack than does a corresponding location on the advancing blade due to the rotation of the blade and the forward motion of the aircraft. More of the retreating blade therefore tries to accelerate the rotor, while more of the leading blade tries to decelerate the rotor. The net result is a balance of forces and a stable rotational speed of the rotor.
Rotation of an aircraft rotor as a result of air moving from the bottom side of the rotor disc to the top side of the rotor disc as in an autogyro is referred to herein as ‘autorotation.’
The Invention is not taught by the prior art of autogyros, helicopters or compound aircraft.